When is it right to back off of the net?

Strategy thing that I'm not really sure of yet - when is it correct to back off of the net somewhat (middle of box to service line for example) if you are up volleying in singles? in doubles?

I think the obvious one is if a lob went bad and they are going to hit an overhead - but not really sure on the other "correct" scenarios. A weak short shot? A weak shot that they are going to have plenty of time to set up for a power shot?

I feel like I move around the box a lot depending on how I feel about the point, but no clue if what I'm doing is correct or if it is just happy feet.

I think too many people underestimate their ability to cover lobs. You don't have to be able to crush a scissors kick overhead from midcourt to be effective. Take it out of the air with a high volley if you have to and then recover forward. I see far too many people around the club turn around and run back when they see a lob even on short/poorly hit lobs. A truly offensive lob is an advanced shot that not many people have in the toolkit.

If you cough up a volley that gives your opponent a sitter, stand your ground and take it. Just keep your racket up and be ready to react. Cut off the angles if you can or take a gamble and guess to cover one side. Maybe you get passed, maybe you'll get drilled, maybe your opponent cuts it too close and misses, maybe you get a racket on it and you volley it back for a winner. If you retreat you are essentially taking yourself out of the point by giving your opponent the whole court to hit into and even if for some reason they hit it to you, if you're moving backwards you won't have your feet/weight behind your shot.